You could always fork the code, I suppose, to eliminate that button from the interface. The fact is the user at the machine can ALWAYS regain control by pulling the network cable, restarting the machine, and/or disabling VNC.

This is somewhat simpler if using a one-time Single Click solution rather than a permanently installed server / service.

(And if you don't trust the person coming in via VNC to <b>not</b> lock you out, why are you letting them control your machine at all?)